In Lehmer et al. U.S. Pat. No. 5,220,361 issued Jun. 15, 1993 entitled Gaze Tracking for Field Analyzer, the inventors herein disclose a method and apparatus for tracking the gaze angle of the human eye during a so-called "field test."
A field analyzer is a device for surveying the sensitivity of a patient's retina. A spot of light, termed a point, is projected to a hemispherical projection screen for a short period of time. A patient viewing the hemispherical projection screen from the center of the sphere fixates along a line of sight to a fixation light source mounted on the surface of the bowl. The point of projection on the hemispherical projection screen controllably changes to positions spaced apart from the fixation light source. Preferably, the point is varied in intensity as the point moves from position to position on the hemispherical projection screen. A subjective determination is made by the patient by depressing a response button FIG. 1A item 30, if the point is seen. By positioning the point to known locations on the hemispherical projection screen and changing the brightness (in a total amount of about four decades), the sensitivity of the patient's retina is measured and mapped.
This simple concept has two basic optical problems interfacing to the patient. First, the patient must fixate on the center of the hemispheric projection screen. This fixation must be maintained when the point is presented, usually to the side of the patient's fixated line of sight, if the point is to fall on a consistent part of the retina. Secondly, the patient's vision usually must be properly corrected to focus the surface of the hemispherical projection screen onto the retina.
It will be understood focus is particularly critical when the sensitivity of the retina is measured at the threshold of the patient's vision perception; were the patient's focus not correct, targets that should be seen are not detected and give erroneous results. This is due to the fact that an unfocused spot of light appears dimmer than a focused one.
The patient's eyeglasses are almost always unsuitable for providing a focused view of the points on the hemispherical projection screen for at least three reasons. First, the frames of the patient's glasses will vary unpredictably in size and shape. They are an unknown in the areas of obscuration of vision and lens tilt angle. Moreover, it is vital that the conditions of testing be repeatable over a period of many years. This would be affected by changes in the patient's eye-wear.
Secondly, the optical prescription within the patient's glasses is almost always deficient for the particular focal distance (usually about 30 centimeters) required for the test. The glasses almost always do not correct the patient's vision to the distance from the patient's eye to the surface of the screen.
Thirdly, the viewing angle of the patient's glasses is usually deficient. For example, the glasses of the patient may contain bifocal lenses or variable lenses which change the focal distance of the patient as a function of the point position on the screen. Where testing of the field of vision of a patient is being made, such glasses give erroneous results.
Because of these limitations, vision during a field test is typically corrected by so-called trial lenses which are selected to provide vision corrected to the 30 centimeter focal distance and placed near the eye in a trial lens holder. Moreover, two lenses are usually required, one to correct spherical power and one to correct cylinder (astigmatic) power.
The correction of the patient's eyesight is accomplished by adding one or two trial lenses to the optical path, directly in front of the patient's eye. These usually round lenses are made in a variety of sphere and cylinder powers and are selected by the operator based upon the patient's prescription, corrected to 30 centimeters, the radius of the hemispherical projection screen.
The standard trial lenses are relatively small in diameter (on the order of 3.5 cm). The center of the trial lenses should be placed in the approximate center of the eye to avoid prismatic effects associated with strong lenses. Additionally, the trial lenses should be close to the eye, to prevent the obscuring of the patient's vision by the trial lens holder or lens frame. Most field testing is done within a 30 degree angle from the fixation axis. Closeness is even more important when strong positive lenses are used as they make the viewing angle through the lenses smaller by magnifying the bowl.
In all known field test devices to date, the position of the lenses is fixed relative to the center of the screen, requiring the position of the patient's eye to also be fixed. This state is monitored by a video camera FIG. 1A item V and presented to the operator as a surveillance tool. Movement of the patient's eye to re-center same in the trial lens may require adjustment of the chin cup by the operator.
Field analyzers typically use the ambient screen light for illumination of the video field. The ambient screen light of most field testers comes from the uniform illumination of the hemispherical projection screen surface, this illumination being provided to give uniform contrast to the projected points. It is also known to illuminate the eye from lights mounted on the trial lens holder using infra-red wavelengths to prevent the patient from detecting the lights.
In addition to the practical mechanical alignment problems attendant upon the use of trial lens, an additional problem exists regarding gaze direction in measuring the sensitivity of the patient's retina during the field test procedure.
Mapping the recognized variably positioned points on the spherical projection screen accurately onto corresponding positions on the retina requires that the eye does not change its angular relationship to the center of the hemispherical projection screen as the test progresses. The eye, however, is disposed in the head in such a way that changing gaze direction is easily accomplished, and in fact is the most natural thing to do when an object--such as a dim spot of light--comes into peripheral view. It therefore requires a great amount of concentration on the part of the patient to maintain a constant gaze direction. In short, the test procedure consuming normally up to 20 minutes for each eye can be very tiring on the patient.
In the normal field test the patient is asked to direct his or her vision straight ahead by "fixating" on an illuminated target. This positions the eye to image the target on the macula portion of the patient's retina, the area of the eye's highest resolution. Fixation on the center of the screen maintains a constant relationship between the points on the screen and specific locations on the retina, even with a change in the patient's head position from the central position.
It is known to check the patient's gaze direction by presenting points at the so-called optical cup or "the blind spot" of the patient's retina to be certain that such points are not seen. It is a well known natural phenomenon that overlying the optic cup on the retina of the normal eye there is an area where light is not seen. Near the beginning of a normal field test, the position of the blind spot is determined by presenting many points near the expected position of said blind spot. It is assumed that the patient is properly fixated at this time. With the position of the blind spot of the patient determined, provision is made to present points periodically to this position in the hemispherical projection screen, which position will be "blind" to the patient's eye. Normally, and assuming the patient maintains correct gaze direction, this periodically presented point is not seen and a negative response is given by the patient to the presentation of the point. A positive response indicates that the patient is not maintaining correct gaze direction at the time of presentation to the "blind spot."
It is to be understood that the presentation of points of light to the blind spot adds time to the test. Additionally, such periodic presentations constitute only a spot check of gaze direction; the patient may have incorrect gaze direction for some interval in the temporal gap between successive spot checks. At present, measuring actual gaze direction is not in common use in commercial field testers.
There are some field test instruments which measure loss of a central pupil position and claim they are measuring gaze direction. This measurement does relate to the trial lens centering issue, reporting the patient is, or is not, centered on the trial lens, but has no bearing on the actual gaze direction. It is to be understood that the eye can be gazing in virtually any angular direction with the pupil perfectly centered in the trial lens.
During field testing, it is known to observe the eye under test in a video presentation. This enables the operator to have a continuous view of the patient's eye position with respect to the trial lens holder to detect obvious deficiencies in the alignment of the patient. Unfortunately, the operator may be either periodically absent or attending to other tasks which divert his or her attention from the video presentation. Further, the operator cannot determine gaze direction from the video display and typically is unaware of when the actual point is presented, the only time when gaze direction is important. Only pupil position can be reliably measured. There is, however, a natural relationship between eye movement activity and the likelihood of satisfactory gaze direction performance. Such video presentations require a video camera and sufficient light for the video presentation to be accurately recorded.
Field analyzers are known that illuminate the hemispherical projection screen with an even field of light generated by incandescent lamps which contain some infra-red energy. Typically, the video camera used is sensitive in the infrared spectrum. This increases the contrast for patients with a dark colored iris between light reflected from the iris and the dark pupil, as all iris colors reflect about the same amount of light with infrared illumination.
However, this illumination system also reflects light from the trial lens surface. The hemispherical projection screen partly surrounds the lens. The lens is typically not anti-reflection coated. Therefore the lens glows with infrared light captured from the hemispherical projection screen. This glow from the lens reduces the pupil to iris contrast in the video image.
In our U.S. Pat. No. 5,220,361 entitled Gaze Tracking for Field Analyzer, we propose automated movement of the trial lens holder to follow the possible movements of the head of the patient during the field test. Additionally, we disclose a method of gaze tracking which includes image subtraction and comparison of the gaze by looking at the relative positions of a reflection and the center of the pupil. This application relates to improvements over these specific techniques. To avoid unnecessary duplication, our U.S. Pat. No. 5,220,361 entitled Gaze Tracking for Field Analyzer is incorporated herein by reference.